After dining off ironing boards at Bakesale Betty's in Oakland, I thought I knew all about casual ambience. But then, heading up the coast on the east side of Tomales Bay, I came upon the Marshall Store, where tables are made of boards on wine barrels. Lawn-chair seating provides a nice view of the Bay and the Point Reyes National Seashore beyond the water. Tomales Bay oysters are sold here raw on the half shell or sizzled momentarily on the grill with butter, then a dab of barbecue sauce. The Marshall Store's oyster chef told us that if you cook the oysters over too hot a flame, they literally will jump out of their shells.